Prolonged Viral Shedding of Influenza Virus: Which Definition?

Influenza virus infection poses a considerable risk for complications to the general population and in particular to solid organ transplant recipients (SOTR). Life-long immunosuppression in SOTR likely contributes to delayed clearance of influenza virus from the airways: Prolonged Viral Shedding (PVS) has important implications for potential infectivity and infection control measures. Duration of infectivity as measured by viral culture has been reported to last 4-6 days in the non-transplant setting. Shedding measured by Polymerase Chain Reaction (PCR) in immune competent patients is similar, 5-6 days. To date there is no recommended or widely accepted definition of PVS for influenza virus infections. The lack of a PVS definition makes comparisons between studies difficult. Most studies assess shedding duration by serial PCR of nasopharyngeal swabs. A number of studies calculate shedding from the time of onset of symptoms to the last positive detection. Shedding is considered to be “prolonged” if it continues on or beyond day 7 or 14. However, considerable variability exists in defining PVS. A large number of studies rely on two objective measures to define the duration of shedding: This requires at least two positive detections of viral material, usually by PCR. We discuss the different aspects of these definitions and propose a practical definition that takes into account a number of factors relevant to the topic.